The present invention relates to a rate of flow controlled automatic, medical breathing apparatus particularly suitable for reanimation and anaesthesia rooms.
The conventional rate of flow controlled automatic, medical breathing apparatuses--also known as lung ventilators--respiratory gas flow depending on the clinical and physical conditions of the patient.
Such breathing apparatuses are conventionally provided with a relatively low pressure source, of the order maximally allowed by the respiratory passages of the patient, and with an inner resistance of the apparatus which varies according to the difference between a theoretical preselected value and a real measured value, both values relating to one or more flow parameters, as disclosed--for example--in the U.S. Pat. No. 3,741,208.
A drawback of this solution resides in the flow instability resulting as a consequence of the occasional changes of the patient's resistance. This is due to the feeble pressure head. It follows that necessity causes an effort to be made to reset the flow by correcting the error by means of devices operated by a step motor which squeeze or open a resilient pipe, usually a silicone rubber pipe, through which the respiratory gas flows.
Since the operating times of such devices are very long in relation to the exigencies of the respiratory cycle (the operating times are of the order of 1/10 second while the total inspiration times are of the order of 2/10 or 3/10 second) the resulting errors are so excessive that effective control of the inspiratory curve is not obtained.
The correction of the error, and hence of the rate of flow curve, further depends on the accuracy of the same rate of flow measurement. The accuracy of such measurement depends upon the accuracy of the transducer used which, as is known, is systematically degraded by the necessity of sterilization of the transducer. It follows that an incorrect measurement is directly reflected on the flow pattern.
A further conventional technique used in the rate of flow controlled breathing apparatus resides in the provision of a high pressure source with elevated internal constant resistance. Thus, when chstable. The drawback of such a type of high pressure ventilators resides essentially in the difficulty of keeping the primary high pressure steady when substantially large drawings of respiratory gas occur. As a consequence, there is a loss in the rate of flow control. High pressure stability and hence stability of the flow can be obtained by using a large high pressure reservoir, but such a large reservoir in turn causes a drawback; variations of the respiratory gas composition imposed at the input are displaced at the output with a too long delay. On the other hand, it is practically impossible to maintain the high pressure steady in a feeding source formed by a bellows subjected to the pressure long, strong and of difficult adjustment and/or an excessively small volume of utilizable respiratory gas would be obtained from the bellows.
In the conventional breathing apparatuses of the above described type the inspiration curve is determined by the structure of the same apparatus and said structure cannot be modified so as to adapt the breathing apparatus to the required various clinical exigencies.